Field of the Invention
The present invention relates to a spread spectrum communication method and a spread spectrum communication apparatus suitable for application to mobile radio communication or the like.
Along with an increase in radio communication stations, the spread spectrum communication system, which is relatively immune from noise and interference, is attracting extensive interest. The spread spectrum communication system is a communication formula whereby the spectrum bandwidth is intentionally expanded by modulating signals, which have undergone usual data modulation, such as PSK or QAM, with a high speed sequence of codes known as spreading codes, and the coding rate of these spreading codes is called the chip rate fc. The chip rate fc has a relationship to the coding rate of the transmit data to be spread, i.e. the bit rate fb, of BW=fc/fb (BW is an integer), and this integral value BW is called the bandwidth expansion factor.
In recent years, techniques to make the aforementioned chip rate fc for communication systems using spread spectrum communication have been proposed. For instance, the Japanese Patent Laid-open No. 8-065264 discloses a method by which, although a plurality of receiving stations use the same spreading codes, each receiving station is enabled, by making the chip rate fc variable, to extract only desired signals by detecting correlations and performing despreading at the same chip rate fc as the counterpart transmitting station.
FIG. 12 is a block diagram illustrating the configuration of the receiving section of the spread spectrum communication apparatus proposed in the Japanese Patent Laid-open No. 8-065264. In this system, the mutually opposite transmitting and receiving stations can choose the type of code from a selective spreading code generator 65 and the chip rate from a selective clock generator 66.
Therefore, by predetermining the code type and the chip rate between the transmitting and receiving stations, only signals from the counterpart in the communication can be extracted at the time of despreading even if the code type of an interfering wave coincides because the chip rate is different.
Further, the Japanese Patent Laid-open No. 6-276176 proposes a method by which, with a view to solving the relative distance problem by reducing inter-signal interference at the time of demodulation due to an imbalance in reception field strength of signals from remote stations at a base station, a lower chip rate fc is given to the transmitting side when receive signals of a high field strength are received by the base station and a higher chip rate fc is given to the transmitting side when receive signals of a low field strength are received so as to achieve the best possible uniformization of reception field strength at the base station.
FIG. 13 is a block diagram illustrating the configuration of the CDMA communication system disclosed in the Japanese Patent Laid-open No. 6-276176. Signals from remote stations 71 and 72 are subjected to despread-demodulation by a despread-spectrum demodulating section 74 in a base station 73 and to receive power determination by a receive power detecting section 75. On the basis of the detected receive power, a chip rate determining section 76 and a chip rate notifying section 77 carry out chip rate control over the aforementioned remote stations.
Whereas the benefit of a variable chip rate fc is as described above, making the chip rate fc variable means making the bandwidth expansion factor BW variable, and making the bandwidth expansion factor BW variable provides the following benefits.
(a) By raising the bandwidth expansion factor BW, the S/N ratio of the desired wave after despreading on the receiving side can be improved.
(b) Raising the bandwidth expansion factor BW results in an expanded bandwidth and a corresponding reduction in transmit peak power, which makes possible suppression of interference with other stations.
(c) The bandwidth expansion factor BW can be so set as to optimize the efficiency of frequency utilization of the whole system.
On the other hand, since the S/N ratio of the largest correlated values obtained by a correlator in a receiver is proportional to the code length (the number of chips per period) L [chips] of spreading codes, the correlation detecting performance of the receiver can be improved by extending the code length L.
Now, in the conventional spread spectrum communication system described with reference to FIG. 12 and FIG. 13, 1 period equivalent of spreading codes is always accommodated within 1 information bit, and the following equation holds.
L=BW=fc/fbxe2x80x83xe2x80x83(1)
In order to improve the correlation detecting performance of a receiver and make it relatively immune from noise and interference, a method to raise the bandwidth expansion factor BW and another to extend the code length L of spreading codes is conceivable. However, where the code length L and the bandwidth expansion factor BW are always kept equal as in the spread spectrum communication systems according to the prior art, it is impossible to make these factors independently variable.
Especially, the code length L of spreading codes cannot be made independently variable in disregard of the bandwidth expansion factor BW, and in almost every case it is limited by the bandwidth expansion factor BW.
The reason is that the bandwidth expansion factor BW is prevented from being raised beyond a certain level by the need to optimize the efficiency of frequency utilization by the whole system in consideration of the environment of use, spread processing and the limitation of the operating speed of a device performing analog-to-digital (A/D) conversion at a later stage, both on the transmitting side, and despread processing and the limitation of the operating speed of a device performing digital-to-analog (D/A) conversion, both on the receiving side.
Therefore, in the conventional spread spectrum communication systems, where the environment of use or the limitation of devices prevents the bandwidth expansion factor BW from being raised substantially, the code length L of spreading codes is kept short, resulting in poor correlation detecting performance of the receiver.
In order to solve this problem, it is necessary to enable 1 period of spreading codes to span a plurality of information bits. Where 1 period of spreading codes spans N information bits, the following equation holds.
N=L/BWxe2x80x83xe2x80x83(2)
What poses a problem here is that the value of N bits is not fixed. Since this value of N bits constitutes an information bit, it is not in a fixed pattern, such as being always xe2x80x9c1xe2x80x9d or the like. Correlation detection at this time is accomplished as represented by the following equation.                               C          ⁡                      (            j            )                          =                              1            L                    ⁢                                    ∑                              k                =                0                                            L                -                1                                      ⁢                                          R                ⁡                                  (                                      j                    -                    k                    +                    1                                    )                                            ·                              pn                ⁡                                  (                  k                  )                                                                                        (        3        )            
In Equation (3), C(j) represents the correlated value at a time j; R(j), the spread receive signal entered into the correlator at the time j; and pn(k), a despreading code.
If transmit and receive codes are identical in timing, and the values of all of N information bits before the spread are either xe2x80x9c1xe2x80x9d or xe2x80x9cxe2x88x921xe2x80x9d, Equation (3) will give a value of xe2x80x9c1xe2x80x9dor xe2x80x9cxe2x88x921xe2x80x9d, respectively.
However, if N information bits before the spread randomly include xe2x80x9c1xe2x80x9d and xe2x80x9cxe2x88x921xe2x80x9d, the result will vary with the ratio between xe2x80x9c1xe2x80x9d and xe2x80x9cxe2x88x921xe2x80x9d at a given time. If, for instance, xe2x80x9c1xe2x80x9d and xe2x80x9cxe2x88x921xe2x80x9d are included in equal proportions, the result will be xe2x80x9c0xe2x80x9d. As a correlator usually recognizes the peak of correlated values as the coincidence of transmit and receive codes in timing, the correlator is unable to correctly detect coincidence in code timing in such a case.
As described above, spread spectrum communication systems according to the prior art involve the problem that they do not allow the code length L of spreading codes and the bandwidth expansion factor BW to vary independently and, if this problem is to be solved, there will arise another problem that the correlator on the receiving side cannot correctly detect timing coincidence.
The present invention, attempted to solve the problems noted above, is intended to provide a spread spectrum communication method and a spread spectrum communication apparatus capable of setting the bandwidth expansion factor so as to achieve the optimal efficiency of frequency utilization according to the environment of use while averting the aforementioned problems occurring on the correlator on the receiving side, and permitting the determination of the code length of spreading codes without having to worry about the efficiency of frequency use and the limit of the operating speed of devices.
According to the spread spectrum communication method of the present invention, a fixed pattern is multiplexed over the leading edge of each frame of transmit data; a spreading code is generated at a timing synchronized with each such frame; the transmit data multiplexed with the fixed pattern is spread-modulated with the spreading codes and transmitted to a counterpart station; correlations between spread-modulated signals received from the counterpart station and the same code sequence as the fixed pattern are detected; if the fixed pattern is detected by the correlation detection, a despreading code is generated at a timing synchronized with the frame; and the spread-modulated signals are despread-modulated with such despreading codes. In this way, according to the invention, transmit data over which a fixed pattern is multiplexed are spread-modulated with spreading codes. In this process, there is no need to make the code length L of the spreading codes identical with the bandwidth expansion factor BW after spreading, but the two factors can be set independent of each other. Further, the leading bit of the fixed pattern for frame synchronization and the leading bit of the data immediately after the fixed pattern are multiplied by the first of the spreading codes. On the receiving side, a spread-modulated fixed pattern in receive data is correlatively detected. Upon detection of the fixed pattern, the generation of despreading codes is reset, and the first of the despreading codes is supplied at the next chip.
Further, for the spreading codes, the code length and the bandwidth expansion factor can be set independent of each other.
Also, the ratio between the code length and the bandwidth expansion factor is variable.
Further, a fixed pattern is multiplexed over the leading edge of each frame of transmit data; spreading codes are generated at a timing synchronized with each such frame; the code length and the bandwidth expansion factor of the spreading codes are controlled to prescribed values; the transmit data multiplexed with the fixed pattern is spread-modulated with the spreading codes, and control signals for the code length and the bandwidth expansion factor are transmitted to a counterpart station along with the spread modulation; correlations between spread-modulated signals received from the counterpart station and the same code sequence as the fixed pattern are detected; if the fixed pattern is detected by the correlation detection, a despreading code is generated at a timing synchronized with the frame; the code length and the bandwidth expansion factor of the despreading codes are controlled in accordance with the received control signals; and the spread-modulated signals are despread-modulated with such despreading codes.
Further, the control of the code length and the bandwidth expansion factor is such that at first the bandwidth expansion factor is set to a prescribed optimal value and the code length to a short value, and then the code length is extended on the basis of the reception characteristics of the counterpart station.
A spread spectrum communication apparatus according to the present invention comprises a multiplexing circuit multiplexing a fixed pattern over the leading edge of each frame of transmit data; a spreading code generating circuit for generating spreading codes at a timing synchronized with the frame; a spreading circuit for spread-modulating the transmit data multiplexed with the fixed pattern with the spreading codes and transmitting them to a counterpart station; a correlation detecting circuit for detecting correlations between spread-modulated signals received from the counterpart station and the same code sequence as the fixed pattern; a despreading code generating circuit for generating, if the fixed pattern is detected by the correlation detection, a despreading code at a timing synchronized with the frame; and a despreading circuit for despread-modulating the spread-modulated signals with the despread-modulated signals. Thus, the spreading circuit of the transmitting station spread-modulates the data entered from the multiplexing circuit with spreading codes entered from the spreading codes generating circuit. In this process, there is no need to make the code length L of the spreading codes identical with the bandwidth expansion factor BW after spreading, but the two factors can be set independent of each other. Further, the leading bit of the fixed pattern for frame synchronization and the leading bit of the data immediately after the fixed pattern are multiplied by the first of the spreading codes. On the other hand the correlation detecting circuit of the receiving station correlatively detects a spread-modulated fixed pattern in receive data. Upon detection of the fixed pattern, the despreading code generating circuit is reset, supplies the first of the despreading codes at the next chip, the despreading circuit despread-modulates the receive data with the despreading codes, and restores the data before the spreading.
Further, for the spreading codes, the code length and the bandwidth expansion factor can be set independent of each other.
Also, the ratio between the code length and, the bandwidth expansion factor is variable.
There is further provided a spread spectrum communication apparatus comprising a multiplexing circuit for multiplexing a fixed pattern over the leading edge of each frame of transmit data; a spreading code generating circuit for generating spreading codes at a timing synchronized with the frame; a spreading code control circuit for controlling the code length and the bandwidth expansion factor of the spreading codes to prescribed values; a spreading circuit for spread-modulating the transmit data multiplexed with the fixed pattern with the spreading codes; a modulating/demodulating circuit for transmitting to a counterpart station control signals for the code length and the bandwidth expansion factor along with the spread modulation; a correlation detecting circuit for detecting correlations between spread-modulated signals received from the counterpart station and the same code sequence as the fixed pattern; a despreading code generating circuit for generating, if the fixed pattern is detected by the correlation detection, a despreading code at a timing synchronized with the frame; and a despreading circuit for despread-modulating the spread-modulated signals with the despread-modulated signals, wherein the spreading code control circuit of the transmitting station controls the code length and the bandwidth expansion factor of the spreading codes to prescribed values while the spreading code control circuit of the receiving station controls the code length and the bandwidth expansion factor of the despreading codes on the basis of the control signals that have been received.